Heretofore, sheet processing apparatuses for counting and distinguishing various sheets such as banknotes have been brought to practical use. A sheet processing apparatus takes in sheets inserted in an insert portion one by one and conveys the sheets to a light detection device. The light detection device detects light from each of the sheets and acquires an image based on the detected light. The sheet processing apparatus determines the type and authenticity of the sheet based on the acquired image. The sheet processing apparatus also determines whether or not the sheet can be recirculated, based on the acquired image. The sheet processing apparatus stacks the sheets in a state in which the sheets are classified by type based on results of the determinations.
On sheets to be processed by the sheet processing apparatus, there are printed fluorescent printed information (first printed information) and phosphorescent printed information (second printed information). The fluorescent printed information is information printed in ink (fluorescent ink) containing fluorescent material. The fluorescent material is excited by excitation light (e.g., ultraviolet rays) or the like emitted from an excitation light source. The excited fluorescent material emits light (fluorescence). When the irradiation with the excitation light is interrupted, the fluorescent material stops emitting fluorescence.
The phosphorescent printed information is information printed in ink (phosphorescent ink) containing phosphorescent material. The phosphorescent material is excited by excitation light (e.g., ultraviolet rays) or the like emitted from an excitation light source. The excited phosphorescent material emits light. When the irradiation with the excitation light is interrupted, the phosphorescent material emits light (afterglow) which gradually attenuates with time.
Japanese Patent No. 3790931, for example, discloses a light detection device including an excitation light source, a sensor for detecting fluorescence, and a sensor for detecting phosphorescence.
In general, a light detection device for detecting fluorescence and phosphorescence includes a fluorescence detecting portion on the upstream side of a conveyor path along which sheets are conveyed. Further, the light detection device includes a phosphorescence detecting portion downstream of the fluorescence detecting portion.
The fluorescence detecting portion irradiates a sheet with excitation light from an excitation light source to excite the fluorescent and phosphorescent materials. The fluorescence detecting portion detects fluorescence emitted from the excited fluorescent material and reads fluorescent printed information.
The phosphorescence detecting portion detects afterglow emitted from the phosphorescent material after interruption of the irradiation with the excitation light by the fluorescence detecting portion, and reads phosphorescent printed information.
The amount of light emitted by the excitation light source changes due to aging. Also, the sensitivities of the sensors of the detecting portions may change. For these reasons, light detection devices have generally been brought to practical use, each of which includes a fluorescent reference plate in the vicinity of a detection position of a detecting portion.
In this case, the fluorescence detecting portion includes a fluorescent glass as a fluorescent reference plate. The fluorescent glass emits fluorescence as a detection reference toward a sensor. The sensor and the fluorescent glass are disposed to face each other across a conveyor path of a sheet processing apparatus. The fluorescence detecting portion detects fluorescence emitted from the fluorescent glass while no sheets are being conveyed. The fluorescence detecting portion corrects the amount of light emitted by a light emitting element or the sensitivity of the sensor based on the value of the detected fluorescence.
However, light from an excitation light source does not reach a detection position of a phosphorescence detecting portion. Accordingly, if a fluorescent glass is disposed at the detection position of the phosphorescence detecting portion, the fluorescent glass is not excited. Thus, there has been the problem that correction cannot be performed on a sensor of the phosphorescence detecting portion.